Methods of manufacturing fissionable materials for use in nuclear reactors



United States Patent 3,177,069 METHODS OF MANUFACTURING FISSIONABLEMATERIALS FOR USE IN NUCLEAR REACTORS Marcel Englander, Paris, andJacques Andr Stohr, Burcs: sur-Yvette, France, assignors to Commissariata lEnergie Atomique, Paris, France No Drawing. Original application June7, 1956, Ser. No. 589,876. Divided and this application Feb. 24, 1960,Ser. No. 10,541 Claims priority, application France, June 14, 1955,693,701, Patent 1,129,082; Apr. 21, 1956, 713,135, Patent 1,149,991

4 Claims. (Cl. 75-12217) The present invention relates to methods ofmanufacturing fissionable materials, including high proportions ofuranium, for use in particular in nuclear reactors.

Pure metallic uranium, when used as fissionable material in a nuclearreactor, has, from the mechanical point of view, drawbacks which are themore important as the flux of neutrons and the power to be supplied arehigher. The fissionable material in the form of bars or slugs isdeformed and there are risks of said material deteriorating theprotective jackets or cans in which said bars are enclosed.

The object of the present invention is to provide a fissionable materialwhich is free from these drawbacks.

For this purpose, according to our invention, such a fissionablematerial is constituted'by an alloy of uranium, at least one firstaddition metal of the group consisting of aluminium, zirconium,chromium, titanium and vanadium and at least one second addition metaldifierent from the first one and belonging to the fifth horizontal lineof the periodical classification of chemical elements, that is .to say ametal of the group consisting of yttrium, zirconium, niobium,molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tinand antimony.

It was found that the presence of this second addition metal greatlyfacilitates the thermal treatment necessary for the obtainment of thedesired material and, in particular, the duration of this treatment,instead of being from 200 to 300 hours when the alloy contains only thefirst addition metal, may be reduced to less than 50 hours owing to thepresence of the second addition metal.

The amounts of the addition metals are low for the first element andstill lower for the second one. Practically they do not exceed 2% byweight for the first one and 1% by weight for the second one,theseweights being based upon that of the pure metallic uranium presentin the alloy.

The second addition metal is advantageously constituted by zirconium (ofcourse when the first addition metal does not consist of zirconium); itsamount is then about 0.1% by weight. We may also use molydenum as secondaddition metal, its amount then ranging from 0.5% to 1% by weight.Advantageously, the second ad dition metal is added in'the form of analloy of said metal with uranium. 7

According to a first embodiment of the method according to ourinvention, the addition metals are added to pure metallic uranium, inone or several steps, for in stance by, first forming an alloy ofuranium with the first addition metal and adding the second additionmetal to this alloy. The whole is melted under a vacuum averaging from10- to 10' mm. of mercury. The molten :bath is then heated to atemperature averaging 1,500 C.

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the pure state, or in the state of a combination that is reducible bycalcium, magnesium or any other metal capable of reducing uraniumtetrafluoride. This mixture of uranium tetrafiuoride and the first,addition metal is then subjected to the action of calcium, magnesium orsaid other metal to reduce uranium tetrafluoride. The suitable amount ofthe second addition metal is added to fatigue.

the alloy of uranium and the first addition metal thus obtained. Thewhole is then treated in the above mentioned'way.

The alloy obtained by either of the above described methods is a twophase alloy, with coarse grains, which has very serious advantages overpure uranium, from the points of view of mechanical characteristics andthermal However, the coarse dispersion of the second phase causesmechanical or other sensitizing effects to exist at certain points ofthe mass of the alloy, and it is necessary to elrninate such effects bya suitable thermal treatment.

For this purpose, the alloy thus obtained is heated to a temperaturesuch that, after a sutficient time and without apparent deformation, alltraces of the second phase which caused said sensitizing effect seem todisappear. This treatment is preferably performed, according to theinvention, under a vacuum averaging 10- mm. of m'ercury in order toavoid any oxidation, at a temperature higher than 1000 C. and lower thanthe temperature at which the alloy starts .rnelting. The alloy is thensuddenly quenched in a cold oil bath, also under vacuum.

This treatment for solubilizing or homogenizing the solid phase, whichdoubles the hardness of the alloy, gives it a crystalline structure of atransitory character which must be transformed into a stable structureby an annealing treatment.

This annealing treatment consists, according to the present invention,in heating the metal mass to a temperature near the upper limit of therange of stability of the orthorhombic phase of uranium (i.e. about 660C.) for a time sufiicient to permit a crystalline re-arrangement of thematrixor uranium phase and a very fine and very regular precipitation ofthe second phase. This further treatment then gives the alloy a stillhigher hardness and a better resistance to thermal fatigue than that ofthe solubilized alloys.

We will now give examples of the method according to our invention.

In a first example, a billet of uranium of about 50 kgs. contains, in arecess provided for this purpose about 0.4% of its weight of aluminiumshot in a state of purity of 99.99% and 0.15% of zirconium in the formof a uranium-zirconium alloy containing 2.5% of zirconium.

This billet having its recess thus filled is heated under a vacuum, forinstance by induction at high frequency (from 4 kc. to 800 kc. forinstance), in a retort protected in such manner that the corrosiveaction by molten uranium is limited to a minimum. This heating isconducted in such manner as always to preserve a vacuum ranging from 10-to 10- mm. of mercury inside the apparatus. After melting, thetemperature is brought to about 1,500 C. for from 30 to 45 minutes, andthe molten mass, still under a vacuum, is poured by means of a funnel,into suitable molds. The casting is effected by gravity. A plug of arefractory material, disposed at the bottom of the retort, is lifted atthe desired time by a pushrod actuated from the outside. The ingots areremoved from the mold after cooling under a vacuum, in order to avoidoxidation of the alloy.

We thus obtain billets of alloy which are subsequently transformed intobars, either by rolling, or by hammering, or by hot drawing, orotherwise. It is also possible directly to obtain bars of a diameteraveraging from 16 to 36 mm. and of a length ranging from30 to 50 cm.

The alloy thus obtained is then subjected, according to the invention,to a first thermal treatment. This treatment consists in heating thealloy under a pressure of about 10'- mm. of mercury for about 50 hoursat a temperature of about 1,050 C. in a resistance furnace provided witha tank which contains oil. In order to prevent an uncontrolledprecipitation during cooling down to the ordinary temperature, the alloyis dropped at the end of the solubilizing treatment, into the cooled oilbath, thus performing a sufliciently sudden quenching to maintain thealuminium is sursaturated solution in the uranium matrix which is thenin a metastable structural state of transitory character.

The morphologic state of this alloy is then stabilized by a new thermaltreatment which consists in heating this alloy under a vacuum at atemperature of about 600 C., for about ten minutes. We thus obtain ametal the Vickers hardness of which averages 420 whereas ordinaryuranium has a hardness of only 210. Our alloy is not deformed whensubjected to thermal fatigue tests.

According to a second example, about 52 kgs. of uranium tetrafluorideare treated according to one of the conventional reduction methods, bymeans of 19 kgs. of calcium to which have been added either about 150grams of aluminium in the form of shavings or 250 grams of powderedalumina.

We thus obtain raw alloy billets which are again molten so as to degasthem, to which there is added 0.15% by weight of n'rconium in the formof an alloy of zirconium and uranium containing about 2.5% by weight ofzirconium.

These billets are then subjected to the thermal treatments describedwith reference to the preceding example.

Of course, our invention, which permits of increasing the duration oflife of the fuel of a nuclear reactor making use of natural uranium oruranium enriched in fissionable elements, is not limited in any way tothe above described examples. It also covers the case where there isadded any metal capable, in small amounts (case of the first additionmetal), of supplying an alloy having an eutectoid reaction, thereforecapable of having its physical and mechanical characteristicssubstantially improved by suitable thermal treatments includingsolubilizing reheatings, quenching and annealings with or withoutmechanical treatment. Addition of the second metal considerably reducesthe duration of said treatments.

In a general manner, while we have, in the above description, disclosedwhat we deem to be practical and elficie-nt embodiments of ourinvention, it should be well understood that we do not wish to belimited thereto as there might be changes made therein without departingfrom the principle of the present invention as comprehended within thescope of the accompanying claims.

The present application is a division of our US. patent: applicationSer. No. 589,876 now abandoned, filed June 7, 1956, for Improvements inFissionable Materials for- Use in Nuclear Reactors and in the Methods ofManufacturing Such Materials.

What we claim is: I 1. The method of manufacturing a fissionablematerial which comprises forming a coarse grain alloy of uranium,

from 0.1 to 2+ percent of the weight of uranium of aluminium, and from0.05 to 1% of the weight of ura nium of zirconium, heating the moltenalloy thus obtained under a vacuum ranging from 10- to 10* mm. ofmercury at a temperature of about l,500 C. allowing said alloy tosolidify, heating it under a vacuum of approximately 10* mm. of mercuryat a temperature higher than 1,000 C. but lower than the melting pointof said alloy, for about 50 hours, quenching suddenly said alloy andannealing it at a temperature of about 600 C.

2. The method according to claim 1 in which an alloy of uranium withaluminium'is first formed and zirconium is then added to this metal.

3. The method according to claim 1 in which said zirconium is added inthe form of an alloy thereof with uramum. 1

4. The method according to claim 1 in which the duration of said firstheating, at about l,500 C., ranges from 30 to 45 minutes and theannealing operation lasts for about 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,830,896 Seybolt Apr. 15, 1958 2,886,430 Allen et al. May 12, 19592,947,621 Foote Aug. 2, 1960 OTHER REFERENCES Proceedings of theInternational Conference on the Peaceful Uses of Atomic Energy; heldinGeneva Aug. 8 to Aug. 20, 1955; vol. 9, pp. 57-67 (recd April 16,1956).

1. THE METHOD OF MANUFACTURING A FISSIONABLE MATERIAL WHICH COMPRISESFORMING A COARSE GRAIN ALLOY OF URANIUM, FROM 0.1 TO 2+ PERCENT OF THEWEIGHT OF URANIUM OF ALUMINUM, AND FROM 0.05 TO 1% OF THE WEIGHT OFURANIUM OF ZIRCONIUM, HEATING THE MOLTEN ALLOY THUS OBTAINED UNDER AVACUUM RANGING FROM 10**-4 TO 10**-5 MM. OF MERCURY AT A TEMPERATURE OFABOUT 1,500*C. ALLOWING SAID ALLOY TO SOLIDIFY, HEATING IT UNDER AVACUUM OF APPROXIMATELY 10**-3 MM. OF MERCURY AT A TEMPERATURE HIGHERTHAN 1,000*C. BUT LOWER THAN THE MELTING POINT OF SAID ALLOY, FOR ABOUT50 HOURS, QUENCHING SUDDENLY SAID ALLOY AND ANNEALING IT AT ATEMPERATURE OF ABOUT 600*C.